Optical heads are used to project light onto an optical medium to record information or to receive reflected light from the optical medium to reproduce information Therefore, the optical heads are provided with an optical path to project light from a light source to the optical medium and an optical path to receive reflected light from the optical medium. In some cases, these optical paths may be separated by using a polarizing diffractive element.
Japanese patent application laid-open No.3-29129(1991) discloses an optical head, as shown in FIG. 1, which is provided with a polarizing diffractive element.
In this optical head, a laser diode 101 as a light source emits light polarized in the direction perpendicular to the surface of this sheet. A polarizing diffractive element 102 transmits more than 20 dB of the polarized light in the direction perpendicular to the surface of the sheet and diffracts more than 20 dB of the polarized light in the direction parallel to the surface of the sheet. The light emitted from the laser diode 101 and then transmitted through the polarizing diffractive element 102 is converted into circular polarization light by a quarter-wave plate 103, converged upon an optical disk 105 by a lens 104. Thereby, the recording of information is conducted. On the other hand, light reflected on the optical disk 105 is reversely passed through the same optical path, converted into light polarized in the direction parallel to the surface of the sheet by the quarter-wave plate 103. The polarized light is diffracted by the polarizing diffractive element 102 into +1st-order diffraction light and -1st-order diffraction light, where the +1st-order diffraction light is received by a photodiode 106 and the -1st-order diffraction light is received by a photodiode 107. Thus, based on the lights received by the photodiodes 106, 107, the convergence position on the optical disk 105 can be detected and controlled and the information recorded on the optical disk 105 can be reproduced.
Meanwhile, the polarizing diffractive element 102, as described in the above application, is an element in which an interference fringe is formed according to the existence of proton-exchanging in a lithium niobate crystal by utilizing the fact that an index ellipsoid is varied by proton-exchanging in the lithium niobate crystal. In the polarizing diffractive element 102, a dielectric film formed on a proton-exchanged region controls a phase difference to a non-proton-exchanged region to be even-numbered times a .pi. radian to the light polarized in the direction perpendicular to the surface of the sheet and to be odd-numbered times a .pi. radian to the light polarized in the direction parallel to the surface of the sheet. Thereby, the light polarized in the direction perpendicular to the surface of the sheet is transmitted and the light polarized in the direction parallel to the surface of the sheet is diffracted.
However, in the conventional optical head, there are some problems caused by the fact that the polarizing diffractive element 102 has to be disposed close to the lens 104. The first and second problems are that the environment resistance is deteriorated and that the manufacturing cost of the polarizing diffractive element is increased. In the composition of the conventional optical head, the photodiodes 106, 107 have to be disposed apart from the laser diode 101 to separate the diffraction light from the light projected from the laser diode 101. Therefore, the diffraction angle of reflected light at the polarizing diffractive element 102 needs to be increased. Also, to obtain a high optical power efficiency, it is required that the polarizing diffractive element 102 transmits the light emitted from the laser diode 101 to the quarter-wave plate 103 at a high efficiency and diffracts the light supplied through the quarter-wave plate 103 to the photodiodes 106, 107 at a high efficiency.
Meanwhile, in the fabrication process of the polarizing diffractive element, which is, as described above, composed of the interference fringe formed by proton exchanging and the dielectric film formed on the proton-exchanged region, the proton-exchanged region and the dielectric film have to be formed by a mask commonly used due to a limitation on alignment accuracy of an aligner. However, since the proton exchanging progresses not only in the direction of thickness but also in the in-plane direction, the width of the proton-exchanged region does not agree with that of the dielectric film. When the fringe pitch is decreased to increase the diffraction angle, the influence by the disagreement in width becomes more serious. As a result, the optical power efficiency will be reduced. Thus, it is impossible to have both a big diffraction angle and a high optical power efficiency. In this case, if the optical power efficiency is sacrificed, the reception of light cannot be properly conducted and reliability in the reproduction of information may be reduced. It may therefore cause a functional failure in the optical head.
Therefore, in the conventional optical heads, the diffraction angle has to be sacrificed. To obtain a predetermined distance between the photodiodes 106 and 107, the distance between the polarizing diffractive element 102 and the photodiodes 106, 107 needs to be increased. Therefore, the polarizing diffractive element 102 needs to be disposed close to the lens 104 since the distance between the lens 104 and the laser diode 101 is previously determined. When the polarizing diffractive element 102 is disposed close to the lens 104, an effective area in the polarizing diffractive element 102 where light is projected needs to be increased. Therefore, the polarizing diffractive element 102 needs to be enlarged since the interference fringe has to be formed on the increased effective area. This causes an increase in the manufacturing cost of the polarizing diffractive element. Also, the environment resistance will be deteriorated since the optical path between the polarizing diffractive element 102 and the laser diode 101 and the optical path between the polarizing diffractive element 102 and the photodiodes 106, 107 are lengthened.
The third problem is that an optical module 108, which is composed of the laser diode 101, photodiodes 106, 107, polarizing diffractive element 102 and quarter-wave plate 103 other than the lens 104, cannot be miniaturized. Namely, when the polarizing diffractive element 102 is disposed close to the lens 104, the distance between the polarizing diffractive element 102 and the photodiodes 106, 107 is increased. Therefore, the optical module 108 must be lengthened in the direction of the optical axis. On the other hand, when the photodiodes 106, 107 are disposed apart from the laser diode 101, the distance between the photodiodes 106 and 107 is increased. Therefore, the optical module 108 must be lengthened also in the direction perpendicular to the optical axis.